This invention relates generally to the suppression of interference amongst communication signals.
Military communications are often conducted in an environment that contains two or more interfering signals. For underwater communications using the very low frequency (vlf) radio band and submerged crossed-loop antennas, only two degrees of spatial freedom exists. As a result, beamforming by itself is only able to cancel a single interferer.
Traditional beamforming techniques are based on steering beams so that interfering signals on the beams have minimum magnitude. Another approach to beamforming for two or more antennas has been through the use of the eigenvalues of the cross-correlation matrix of the antenna signals as the steering directions for the beamformer. This technique works well for one or two interferers, providing that in the case of two interferers, the two interferers are received with powers differing by a factor of two or more.
Adaptive locally optimum processing can also be used to minimize interference, so that weak communication signals can be detected in an interference environment. These techniques process either or both of the amplitude and phase of a complex sample of the received signal. The capability of the processing to mitigate against interference, however, depends on the variability of the amplitude or phase of the interference and not primarily on its power.
Heretofore, there has not been a way to use beamforming together with adaptive locally optimum processing to effectively mitigate interference when two or more interferers are present.
There is thus a need for a practical processor that permits the use of beamforming together with adaptive locally optimum processing to provide effective interference mitigation for processing the outputs of several radio antennas when two or more interferers are present.